The problem of controlling static charge during plastic web manufacturing and transport is well known. Generation and uncontrolled discharge of electrostatic charge can cause a number of serious problems including safety hazards. In the field of imaging, particularly photography, the accumulation of charge on film or paper surfaces leads to the attraction of dirt, which can produce physical defects. The discharge of accumulated charge during or after the application of the sensitized emulsion layer(s) can produce irregular fog patterns or “static marks” in the emulsion. The static problems have been aggravated by increase in the sensitivity of new emulsions, increase in coating machine speeds, and increase in post-coating drying efficiency. The charge generated during the coating process may accumulate during winding and unwinding operations, during transport through the coating machines and during finishing operations such as slitting and spooling.
It is generally known that electrostatic charge can be dissipated effectively by incorporating one or more electrically-conductive “antistatic” layers into the support structure. Typical location of an antistatic layer is an external surface, which comes in contact with various transport rollers. For imaging elements, the antistatic layer is usually placed on the side of the support opposite to the imaging layer.
A wide variety of electrically-conductive materials can be incorporated into antistatic layers to produce a wide range of conductivities. These can be divided into two broad groups: (i) ionic conductors and (ii) electronic conductors. In ionic conductors charge is transferred by the bulk diffusion of charged species through an electrolyte. Here the resistivity of the antistatic layer is dependent on temperature and humidity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, ionic conductive polymers, polymeric electrolytes containing alkali metal salts, and colloidal metal oxide sols (stabilized by metal salts), described previously in patent literature, fall in this category. However, many of the inorganic salts, polymeric electrolytes, and low molecular weight surfactants used are water-soluble and are leached out of the antistatic layers during processing, resulting in a loss of antistatic function. The conductivity of antistatic layers employing an electronic conductor depends on electronic mobility rather than ionic mobility and is independent of humidity. Antistatic layers which contain conjugated polymers, semiconductive metal halide salts, semiconductive metal oxide particles, etc., have been described previously. However, these antistatic layers typically contain a high volume percentage of electronically conducting materials, which are often expensive and impart unfavorable physical characteristics, such as color, increased brittleness and poor adhesion, to the antistatic layer.
A vast majority of the prior art involves coatings of antistatic layers from aqueous or organic solvent based coating compositions. For photographic paper, typically antistatic layers based on ionic conductors, are coated out of aqueous and/or organic solvent based formulations, which necessitate an effective elimination of the solvent. Under fast drying conditions, as dictated by efficiency, formation of such layers may pose some problems. An improper drying will invariably cause coating defects and inadequate adhesion and/or cohesion of the antistatic layer, generating waste or inferior performance. Poor adhesion or cohesion of the antistatic layer can lead to unacceptable dusting and track-off. A discontinuous antistatic layer, resulting from dusting, flaking, or other causes, may exhibit poor conductivity, and may not provide necessary static protection. It can also allow leaching of calcium stearate from the paper support into the processing tanks causing build-up of stearate sludge. Flakes of the antistatic backing in the processing solution can form soft tar-like species, which, even in extremely small amounts, can re-deposit as smudges on drier rollers eventually transferring to image areas of the photographic paper, creating unacceptable defect.
Moreover, majority of antistats on current photographic paper products lose their electrical conductivity after photographic processing due to their ionic nature. This can cause print sticking after drying in the photoprocessor, and/or in a stack.
Besides antistatic properties, an auxiliary layer in a photographic element maybe required to fulfill additional criteria depending on the application. For example for resin-coated photographic paper, the antistatic layer if present as an external backing layer should be able to receive prints (e.g., bar codes or other indicia containing useful information) typically administered by dot matrix printers and to retain these prints or markings as the paper undergoes processing. A vast amount of photographic paper in the market uses colloidal silica based antistatic backings, which without a suitable polymeric binder provide poor post-processing backmark retention qualities.
In U.S. Pat. Nos. 6,197,486 and 6,207,361, antistatic layers have been disclosed which can be formed through the (co)-extrusion method thus eliminating the need to coat the support in a separate step and rendering the manufacturing process less costly.
However, there is still a need for electrical conductivity in the antistatic layer than is superior to that currently available. For most paper based imaging products a backside surface electrical resistivity or SER of 13 log ohms/square is considered sufficient for most practical purpose. This is because the paper base itself is ionically conductive, due to the presence of salt and base moisture in these supports, and minimizes the conductivity requirement for the back surface. However, as the next generation of “all plastic” imaging display products are designed utilizing voided polymeric materials, eliminating paper cores such as in U.S. Pat. Nos. 6,093,521; 6,083,669; 6,080,532; 6,074,793; 6,074,788; 6,071,680; and 6,048,606, the conductivity derived from paper cores is lost. Because of their “all plastic” nature, these new products are highly insulating and require higher level of static protection. For such products, backside SER significantly lower than 13 log ohms/square may be necessary for their manufacturing and end use.